The desired signal (the “information”) that is to be transmitted must be converted into a suitable format for transmission. For this purpose, a so called carrier signal is altered by a desired signal. This proceeding is called modulation. The reverse operation, i.e. the filtering out of a desired signal from a carrier signal, is called demodulation. A system that demodulates as well as modulates is referred to as a modem.
The binary transmission of digital signals is thereby achieved, in the simplest case, when a two level square wave signal is used. In this case, it can be switched between two amplitudes, frequencies or phases. The present invention is related to the application of amplitude modulation. In the transmission of digital signals, one speaks of keying rather than of modulation, and in the sense of the invention, of amplitude shift keying (ASK).
Inductively coupled plug-and-socket connections, such as those that are proffered under the designation “Memosens” by the applicant, are, by way of example, suitable for this sort of data transmission.
Inductively coupled plug-and-socket connections are employed in practice, if electrical signals are supposed to be transmitted without electrical contact. Through this galvanic isolation, advantages reveal themselves with regards to corrosion protection, isolation of potentials, prevention of mechanical wear of the plug, etc.
The inductive interface is usually embodied as a system with two solenoids, which are, by way of example, inserted into each other. Typically, data as well as energy is transmitted.
According to the prior art, data sent by means of ASK is extracted in the process of demodulation, by way of example, by an envelope detector and a subsequent comparator, often with hysteresis. In FIG. la, the upper signal threshold of the comparator is designated as A.1 and the lower signal threshold of the comparator is designated as A.2. Above A.1 is detected as logic “high” or “1”, and underneath A.2 is detected as logic “low” or “0”.
As mentioned above, the transmission occurs in the form of, in the ideal case, a square wave signal modulation, as is shown by the reference character B in FIG. 1a. A simple and proven method for the generation of a signal modulation is the application of switching means. High frequency signal components can be generated with each level-change via the circuit components used at the two solenoids for the generation of the signal modulation, especially when using high slew rate switching means. Of particular importance are the signal components that are in the region around the characteristic resonance frequencies of one of the two solenoids that can for example arise from parasitic capacitance layer between the windings. These noise signals have the form of an overdamped oscillation, as is shown in FIG. 1a by the reference character C.1 for a falling edge and C.2 for a rising edge. For these overdamped oscillations, an envelope can be defined that characterizes the damping of the noise signal.
It is possible for these high frequency noise components to interfere with the desired signal, thereby distorting and preventing a successful transmission. This problem is illustrated in FIG. 1a with the desired accurate signal sequence (D.1) and the inaccurately detected signal sequence (D.2): For the level-change “high-low”, the comparator initially, and accurately, detects “0”. However, in the course of time, the noise components of the desired signal oscillate up past the upper signal threshold A.1 of the comparator, which then in the meantime inaccurately registers a “1” and consequently the desired signal is inaccurately transmitted. The analogous goes for a “low-high” level-change.
Furthermore, the noisy signal components, with respect to their frequencies, can be in the region of the frequency band used for the amplitude modulation.